1 .. SPDX-License-Identifier: GPL-2.0
3 .. _transmitter-receiver:
5 Pixel data transmitter and receiver drivers
6 ===========================================
8 V4L2 supports various devices that transmit and receive pixel data. Examples of
9 these devices include a camera sensor, a TV tuner and a parallel, a BT.656 or a
10 CSI-2 receiver in an SoC.
15 The following busses are the most common. This section discusses these two only.
20 CSI-2 is a data bus intended for transferring images from cameras to
21 the host SoC. It is defined by the `MIPI alliance`_.
23 .. _`MIPI alliance`: https://www.mipi.org/
28 The parallel and `BT.656`_ buses transport one bit of data on each clock cycle
29 per data line. The parallel bus uses synchronisation and other additional
30 signals whereas BT.656 embeds synchronisation.
32 .. _`BT.656`: https://en.wikipedia.org/wiki/ITU-R_BT.656
37 Transmitter drivers generally need to provide the receiver drivers with the
38 configuration of the transmitter. What is required depends on the type of the
39 bus. These are common for both busses.
44 See :ref:`v4l2-mbus-pixelcode`.
49 The :ref:`V4L2_CID_LINK_FREQ <v4l2-cid-link-freq>` control is used to tell the
50 receiver the frequency of the bus (i.e. it is not the same as the symbol rate).
52 ``.s_stream()`` callback
53 ^^^^^^^^^^^^^^^^^^^^^^^^
55 The struct struct v4l2_subdev_video_ops->s_stream() callback is used by the
56 receiver driver to control the transmitter driver's streaming state.
59 CSI-2 transmitter drivers
60 -------------------------
65 The pixel rate on the bus is calculated as follows::
67 pixel_rate = link_freq * 2 * nr_of_lanes * 16 / k / bits_per_sample
71 .. list-table:: variables in pixel rate calculation
74 * - variable or constant
77 - The value of the ``V4L2_CID_LINK_FREQ`` integer64 menu item.
79 - Number of data lanes used on the CSI-2 link. This can
80 be obtained from the OF endpoint configuration.
82 - Data is transferred on both rising and falling edge of the signal.
84 - Number of bits per sample.
86 - 16 for D-PHY and 7 for C-PHY
90 The pixel rate calculated this way is **not** the same thing as the
91 pixel rate on the camera sensor's pixel array which is indicated by the
92 :ref:`V4L2_CID_PIXEL_RATE <v4l2-cid-pixel-rate>` control.
94 LP-11 and LP-111 states
95 ^^^^^^^^^^^^^^^^^^^^^^^
97 As part of transitioning to high speed mode, a CSI-2 transmitter typically
98 briefly sets the bus to LP-11 or LP-111 state, depending on the PHY. This period
99 may be as short as 100 µs, during which the receiver observes this state and
100 proceeds its own part of high speed mode transition.
102 Most receivers are capable of autonomously handling this once the software has
103 configured them to do so, but there are receivers which require software
104 involvement in observing LP-11 or LP-111 state. 100 µs is a brief period to hit
105 in software, especially when there is no interrupt telling something is
108 One way to address this is to configure the transmitter side explicitly to LP-11
109 or LP-111 state, which requires support from the transmitter hardware. This is
110 not universally available. Many devices return to this state once streaming is
111 stopped while the state after power-on is LP-00 or LP-000.
113 The ``.pre_streamon()`` callback may be used to prepare a transmitter for
114 transitioning to streaming state, but not yet start streaming. Similarly, the
115 ``.post_streamoff()`` callback is used to undo what was done by the
116 ``.pre_streamon()`` callback. The caller of ``.pre_streamon()`` is thus required
117 to call ``.post_streamoff()`` for each successful call of ``.pre_streamon()``.
119 In the context of CSI-2, the ``.pre_streamon()`` callback is used to transition
120 the transmitter to the LP-11 or LP-111 state. This also requires powering on the
121 device, so this should be only done when it is needed.
123 Receiver drivers that do not need explicit LP-11 or LP-111 state setup are
124 waived from calling the two callbacks.
126 Stopping the transmitter
127 ^^^^^^^^^^^^^^^^^^^^^^^^
129 A transmitter stops sending the stream of images as a result of
130 calling the ``.s_stream()`` callback. Some transmitters may stop the
131 stream at a frame boundary whereas others stop immediately,
132 effectively leaving the current frame unfinished. The receiver driver
133 should not make assumptions either way, but function properly in both